Getting your LED light bar to turn on reliably starts with understanding how the relay in the wiring harness functions as the critical control switch. It’s the component that allows a small current from your vehicle’s cabin switch to safely manage the large current the light bar demands, protecting your dashboard wiring from overheating and potential fire hazards. A standard 20-inch light bar can draw around 10-15 amps; trying to run that directly through a simple switch would melt it. The relay handles this heavy lifting, ensuring power comes directly from the battery through appropriately gauged wires.
Let’s break down the four key terminals you’ll find on a standard automotive relay, typically numbered 85, 86, 87, and 30.
- Terminal 85 (Ground): This wire connects to the vehicle’s chassis or the battery’s negative terminal. It completes the circuit for the switch side of the relay.
- Terminal 86 (Switch Power): This is the “trigger” wire. It receives a 12V signal from the switch you install inside your cab. This low-current signal is what activates the relay’s electromagnetic coil.
- Terminal 87 (Power to Light Bar): This is the output terminal. Once the relay is triggered, it connects terminal 30 to terminal 87, sending high-amperage power from the battery directly to your light bar.
- Terminal 30 (Battery Power): This is the high-current input. It should be connected through an inline fuse directly to the positive terminal of your vehicle’s battery.
For a visual guide to this setup, you can refer to this detailed led light bar relay wiring diagram.
Choosing the Correct Wire Gauge for Safety and Performance
Using wires that are too thin is one of the most common and dangerous mistakes in DIY light bar installations. The wire gauge must be capable of handling the maximum current your light bar will draw without excessive voltage drop over the distance from the battery. Voltage drop translates to dimmer lights and wasted energy as heat in the wires. For most single-row LED light bars drawing under 20 amps, a 14-gauge wire for the main power and ground is sufficient. For larger, dual-row bars or multiple lights, you should step up to 12-gauge or even 10-gauge wire.
| Light Bar Amperage (Estimated) | Recommended Wire Gauge (for runs under 10 feet) | Recommended Inline Fuse Size |
|---|---|---|
| Up to 10 Amps | 16 AWG | 15 Amp |
| 10 – 15 Amps | 14 AWG | 20 Amp |
| 15 – 20 Amps | 12 AWG | 25 Amp |
| 20 – 30 Amps | 10 AWG | 35 Amp |
The Non-Negotiable Role of Fusing
The inline fuse placed within 18 inches of the battery connection on the power wire (to terminal 30) is your primary safety device. It’s not there to protect the light bar; it’s there to protect your vehicle from an electrical fire. If a wire chafes and shorts to the metal chassis, an immense amount of current will flow instantly, heating the wire to the point of melting its insulation and igniting anything nearby. The fuse is the weakest link, designed to blow and break the circuit in milliseconds, preventing catastrophe. Always use the fuse size recommended by the light bar manufacturer or, if unavailable, one slightly above the light bar’s maximum amperage draw.
Step-by-Step Wiring Procedure
Here is a systematic approach to wiring your harness for a clean, professional, and safe installation.
Step 1: Mount the Relay. Find a dry, cool location under the hood to mount the relay. Use a supplied bracket or zip-ties to secure it away from moving parts, excessive heat (like exhaust manifolds), and moisture. A good spot is often on the inner fender wall.
Step 2: Connect the Battery Power. Run a wire of the correct gauge (see table above) from the positive battery terminal to the relay’s terminal 30. Install an appropriate inline fuse holder on this wire as close to the battery as possible, but do not insert the fuse yet.
Step 3: Ground the Harness. Connect the wire from terminal 85 to a clean, unpainted metal part of the vehicle’s chassis or directly to the negative battery terminal. Scrape away any paint or rust to ensure a solid metal-to-metal connection for a reliable ground.
Step 4: Route the Switch Wires. Carefully route the thin-gauge wires from the relay (terminal 86) and the switch’s LED indicator light through the vehicle’s firewall into the cabin. Use existing grommets whenever possible to avoid damaging factory wiring. Never poke a new hole without installing a proper grommet to protect the wires.
Step 5: Wire the Interior Switch. Connect the switch wire (from relay terminal 86) to one terminal of your dashboard switch. Connect the other terminal of the switch to a fused 12V ignition source or a constant 12V source, depending on when you want the switch to have power. The switch’s LED wire will also need a ground connection inside the cabin.
Step 6: Finalize Connections. Double-check that all connections are tight and secure. Ensure wires are routed clear of sharp edges and hot surfaces. Now, insert the correct fuse into the inline fuse holder near the battery.
Troubleshooting Common Issues
Even with careful planning, you might run into problems. Here’s how to diagnose them.
Problem: Light bar doesn’t turn on.
- Check the Fuse: The fuse near the battery may be blown. Replace it with one of the same amperage.
- Test for Power: Use a multimeter to check for 12V at terminal 30 of the relay (with the fuse installed). If there’s no power, trace the wire back to the battery.
- Check the Ground: Test the ground connection at terminal 85. A poor ground is a very common cause of failure.
- Listen for the Click: Have a helper flip the switch while you listen to the relay. You should hear a distinct “click” sound. If you don’t, the problem is with the switch or its wiring (terminals 86 and 85).
Problem: Light bar flickers or is dim.
- Voltage Drop Test: Measure the voltage at the light bar’s connector while it’s on. It should be within 0.5 volts of the battery voltage. A larger drop indicates undersized wiring or a poor connection.
- Inspect Connections: A loose or corroded connection, especially at the ground point, can cause high resistance, leading to dim lights.
Problem: The switch’s indicator light stays on even when the light bar is off.
- Relay Type: This usually indicates you have a “universal” 5-pin relay instead of a standard 4-pin. The fifth pin (87a) is for a normally closed circuit. You need a standard 4-pin SPST (Single Pole, Single Throw) relay.